1. Field of the Invention
This invention relates in general to the preparation of water soluble salts of carboxyalkoxy succinic acids and in particular to the preparation of said salts in solid particulate form with controllable production of high purity tetrahydrate salt.
2. Description of the Art
It is known that water soluble salts of carboxyalkoxy succinic acids are useful detergent builder materials as disclosed in U.S. Pat. No. 3,635,830. The preparation of such materials is described in U.S. Pat. No. 3,914,297.
Where the receovery of salts from alcoholic aqueous solutions has been considered in the past, generally it has been desired to carry out the process at a low temperature, e.g. room temperature or 25.degree.-30.degree. C to minimize alcohol losses by evaporation, to avoid the need for pressure equipment and to minimize combustion hazards.
Carboxymethoxy succinic acid salts, for example, are readily produced by reacting salts of glycolic acid and maleic acid in an aqueous medium in the presence of enough zinc or alkaline earth metal hydroxide typically, calcium hydroxide, to produce a salt system having a pH of at least about 8 and preferably higher than about 11 when measured at 25.degree. C. The reaction is preferably conducted at reflux temperature at about atmospheric pressure for from about 1 to about 6 hours. To produce water soluble salts, typically the calcium salt system is reacted with an alkali metal carbonate such as sodium or potassium carbonate at a moderate temperature of about 60.degree. C and filtered to remove by-product calcium carbonate to yield an aqueous solution containing trisodium carboxymethoxy succinate or tripotassium carboxymethoxy succinate as appropriate. Such solutions can also be produced in other ways such as by converting the zinc or alkaline earth metal salt into acid by ion exchange or treatment with mineral acid and then reacting the carboxyalkoxy succinic acid with an organic or inorganic base to produce other water soluble salts typified herein such as alkali metal, ammonium, or alkanol ammonium salts.
Recovery of water soluble carboxymethoxy succinic acid salts in particulate form from aqueous solutions containing them is difficult, especially on a commercial scale. The usual or frequent result of attempting to recover particulate solid salt from solution by simple procedure as for example by crystallization and drum drying or by spray drying to remove water is the production of a mixture of hydrates which is excessively contaminated with undesirably large quantities of residual reactants an by-product materials.
None of the known prior art describes a suitable process that can be used on a commercial scale for recovering a uniform particulate tetrahydrate product substantially devoid of free water. Especially is it evident that none of the open literature describes how one may obtain selectively tetrahydrate salts of alkali metal carboxyalkoxy succinic acids which are desirable in that they contain less water of hydration than corresponding pentahydrate salts. As far as is known, no open literature teaches how to control the course of precipitation of salts of carboxyalkoxy succinic acids from alcoholic aqueous solution so as to obtain substantially pure tetrahydrate salt. As a matter of fact, the open literature does not even appreciate the tetrahydrate-pentahydrate mixture problem, much less attempt to provide a solution to it.